The present invention relates to the field of thermoforming containers, specifically thermoforming sidewalls of containers, such as cups and the like. More specifically, the present invention relates to improved strength in thermoformed container sidewall designs.
The use of disposable containers by consumers at the workplace, in public areas such as parks, beaches, campgrounds, and the like, and even in the home is a growing practice. For many years, thermofoam materials (e.g., Styrofoam(copyright)) and insulated paper were the only alternatives to glass or re-usable plasticware containers. However, over the last ten or more years, thermoformed plastic model containers have helped to replace the less environmentally friendly foamed containers in the industry for use with hot and cold contents.
Thermoplastic materials are particularly advantageous because, for manufacturers, they are usually of a single construction, do not require expensive foaming agents, and need no surface laminationxe2x80x94each of which is a feature resulting in less steps in the manufacturing processxe2x80x94and, for consumers, they are generally more durable than paper, still inexpensive and recyclable, and in some cases they can even be washed and reused several times before discarding.
Thermoforming begins with a thin sheet or web of material such as polyethylene, polypropylene, polyester, and polystyrene having a thickness within the range of from about 8 mils to about 100 mils, depending on the size of the container to be made. The sheet or web is heated to a temperature suitable for thermoforming the webxe2x80x94in the range of from about 110xc2x0 C. to about 200xc2x0 C. for the above-mentioned materialsxe2x80x94and is then fed into a conventional forming machine with the aid of which the thermoforming process takes place under applied vacuum conditions. A mold cavity is used to impart a particular design into the sidewall of the thin-walled container as the plastic material is drawn into the mold using vacuum pressure on one side and a positive pressure on the opposite side of the material. The sidewall design, if any, is often decorative, but may have a secondary function as well (e.g., texturing for grip, rounded shoulders and recesses for stacking, etc.). The processing time for a normal thermoforming operation is typically between 1 and 20 seconds.
One problem with thermoformed plastic containers, however, particularly with respect to drinking cups, is sidewall integrity. Prior art cups are typically made from plastic sheets having a pre-thermoforming thickness of between 30 to 60 mils, but may be thinner after thermoforming. Picking up such a thin-walled cup when filled with a staining colored or hot liquid can sometimes lead to messy or dangerous spills. The problem is with the sidewall design of the container, which is easily deflected inward constricting the volume of the container. Thicker sidewalls may provide a solution, but at the expense of cost or profit margins.
The present invention provides an economical solution. The present invention is directed to providing a suitable sidewall design for thermoformed containers, especially cups, having an improved strength to weight ratio.
In accordance with the present invention, a new container sidewall, such as for a cup or the like, made of a thermoplastic is disclosed. The container is comprised of a closed bottom wall at one end, an open top at the opposite end, and a sidewall extending between the bottom wall and the open top to define a height. The sidewall has a top portion adjacent the open top of the container, a first set of annular ribs disposed adjacent the top portion, a second set of annular ribs, a middle portion disposed between the first set and second set of annular ribs, and a bottom portion disposed adjacent the closed bottom wall of the container.
In accordance with another aspect of the present invention, the number of ribs in the second set of annular ribs is equal to the number of ribs in the first set of annular ribs. It is an aspect of the invention to provide a container wherein the width of the first set of annular ribs is equal to the width of the second set of annular ribs and each makes up from about 5% to about 40% of the height of the sidewall and wherein the number of ribs in the first and second sets of annular ribs is preferably within the range of from 2 to 10 ribs, and most preferably in the range of from 2 to 5 ribs. Each rib is preferably continuous about the sidewall of the container.
These and other aspects of the present invention set forth in the appended claims may be realized in accordance with the following disclosure with particular reference to the accompanying drawings.